Magnetic star reveals its hidden power

14 August 2013

Artist's impression of a magnetar

Credit: ESA/ATG Medialab

A team of astronomers including two researchers from UCL’s
Mullard Space Science Laboratory has made the first ever measurement of the
magnetic field at a specific spot on the surface of a magnetar. Magnetars are a
type of neutron star, the dense and compact core of a giant star which has
blasted away its outer layers in a supernova explosion.

Magnetars have among the strongest magnetic fields in the
Universe. Until now, only their large scale magnetic field had been measured.
However, using a new technique and observations of a magnetar in X-rays, the
astronomers have now revealed a strong, localised surface magnetic field on
one.

A regular bar magnet has much the same type of magnetic field as the global magnetic field of a magnetar: a dipole. In addition, magnetars have a powerful and heavily warped magnetic field trapped beneath their surfaces.

UCL MAPS/Wikimedia Commons

Magnetars are very puzzling neutron stars. Astronomers
discovered them through their unusual behaviour when observed in X-ray
wavelengths, including sudden outbursts of radiation and occasional giant
flares. These peculiar features of magnetars are caused by the evolution,
dissipation and decay of their super-strong magnetic fields, which are hundreds
or thousands of times more intense than those of the more common type of
neutron stars, the radio pulsars.

The magnetic field of a magnetar can have a complex
structure. The most obvious, and easy-to-measure, component is the large scale
external magnetic field, which is shaped (and behaves) much like a regular bar
magnet’s. This is known as the dipolar field.

The study was carried out on a magnetar called SGR
0418+5729. A few years ago, this star was discovered to have a relatively gentle
dipolar magnetic field compared to other magnetars. However, the star was
showing the typical flaring and bursting activities seen in other
magnetars, leading scientists to suggest
that the star’s magnetic activity might be caused by a field hidden beneath its
surface.

This new study, based on observations from ESA's XMM-Newton
X-ray space telescope, has finally found evidence that SGR 0418+5729 is indeed
concealing a very strong magnetic field in its interior.

“This magnetar has a strong
magnetic field inside it, but it is hidden beneath the surface. The only way
you can detect that is to find a flaw on the surface, where the concealed
magnetic field can leak out,” says Silvia Zane (UCL Mullard Space
Science Laboratory), one of the co-authors of the study.

Such magnetic leaks would also explain the outbursts and
flares of radiation observed from magnetars. The warped magnetic field trapped
inside the star builds up stress below the magnetar's surface, occasionally
breaking its ‘crust’ apart and releasing sudden flashes of X-rays.

Sometimes, the surface breaks and the hidden magnetic field leaks out (artist's impression)

Credit: ESA/ATG Medialab

Magnetars are far too small – only around 20km across – and
distant for even the best telescopes to see any details on their surfaces. They
appear just as dots of light in astronomers’ observations. So the team had to
look for indirect signs of variation on SGR 0418+5729’s surface. To do this,
they measured how the magnetar’s X-ray emissions varied as the star rotates.

“SGR 0418+5729 rotates once
every 9 seconds. We found that at one point during the rotation, the magnetar’s
X-ray brightness drops sharply. That means something on or near one part of the
surface is absorbing the radiation,” adds Roberto Turolla (an honorary
professor at MSSL and co-author of the study).

The team believes that a concentration of protons over a small
area of the magentar’s surface – perhaps as little as a few hundred metres
across – is absorbing the X-rays. The protons are confined to a small volume
near the surface by a strong, localised magnetic field emerging from the
magnetar's interior, giving powerful evidence that a strong and twisted
internal magnetic field lurks beneath the surface.

Closeup of the magnetic field leaking out of a magnetar (artist's impression)

Credit: ESA/ATG Medialab

“This exciting discovery also confirms that, in principle,
other pulsars with relatively low external magnetic fields might conceal a
similar strong magnetic field in the interior. As a result, many pulsars may
switch on and become active flaring magnetars for a while, so in the future we
may discover much more magnetars than what we previously thought. This call for
a major revision of our current ideas of magnetic field formation and
amplification in neutron stars,” explains Zane.

The study is published in the journal Nature.

XMM-Newton is a flagship of the European Space Agency.
The satellite was built and designed by
an international consortium which included UCL Mullard Space Science
Laboratory.

Notes

The research appears in a paper published in the journal Nature, entitled "A variable absorption feature in the X-ray spectrum
of a magnetar"

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